Within the electronics industry, there is a trend towards automated inspection of a circuit board following placement of components on it to detect if any of the components have been improperly placed or are missing. Further, efforts are now underway to inspect the circuit board before component placement but immediately after application of a layer of solder paste, which is applied to bond the components, to detect if the paste has been properly applied. By inspecting the circuit board at each stage of its fabrication, the board can be repaired to correct such defects more economically. For example, if the solder paste has been applied improperly, it is far easier, and hence less expensive, to clean the board and reapply paste to it prior to placement of the components rather than afterwards. Similarly, missing and misplaced components can be corrected more easily if such defects are detected prior to reflow of the solder paste to solder bond the components to the circuit board rather than after reflow of the paste.
In U.S. Pat. No. 4,811,410, issued on Mar. 7, 1989, in the name of I. Amir et al. and assigned to AT&T (incorporated by reference herein), there is disclosed a technique for inspecting a circuit board by directing a beam of light at the board along a first angular direction. The intensity of the light reflected from the circuit board along a second angular direction is sensed by a linescan-type camera. This type of camera is characterized by a plurality of light-sensing elements arranged in a linear array so that each element of the camera senses the light intensity from a separate one of a plurality of small regions which collectively comprise a thin strip of surface area running across the board surface. A relative motion is imparted between the circuit board and linescan camera so the camera captures the image of successive strips of area on the circuit board. The output signal of the linescan camera is then processed to retain only the image of those regions of interest in each strip which is then subsequently processed to detect if defects, such as missing and misplaced components, are present.
The linescan inspection system disclosed in the Amir et al. patent (incorporated by reference herein) has proven useful for pre-solder inspection of circuit boards containing through-hole components (i.e., components with leads designed for insertion through openings in the board). The Amir et al. system, especially when operated in the manner taught in U.S. Pat. No. 4,929,845, issued on May 29, 1990, in the name of I. Amir et al. and assigned to AT&T (herein incorporated by reference), has also proven useful for inspecting circuit boards containing surface mount components. Surface mount components differ from their through-hole counterparts in that surface mount components have conductive members (i.e., leads or pads) designed for solder bonding to metallized areas on the surface of the circuit board. Unlike through-hole components, which are commonly wave soldered to the circuit board, surface mount components are solder bonded to the board by a different process. First, solder paste is applied to the metallized areas on the circuit board and then the conductive members of each the component are placed on the paste-coated areas. After component placement, the solder paste is reflowed to bond the conductive members of the component to the circuit board.
While the Amir et al. linescan inspection system can accomplish pre-solder inspection of circuit boards containing surface mount components, the results are usually less accurate than with circuit boards carrying through-hole components. One reason is that the solder paste applied to the metallized areas on the circuit board tends to be reflective, although much less so than the leads on a leaded surface mount component. In the absence of a component lead, the solder paste thus reflects light into the linescan camera, causing the linescan camera to mistake the paste for a component lead. Errors caused by such spurious light reflection could be eliminated if the presence and nominal location of the components were known from a three-dimensional image of the circuit board. Having a three-dimensional image of the circuit board would also greatly facilitate inspection of circuit board immediately after application of solder paste to determine if the proper volume of paste was applied. Post-solder inspection would also be aided by having a three-dimensional image of the circuit board as well.
In my U.S. Pat. No. 4,965,665, issued on Oct. 23, 1990, and assigned to the same assignee, a technique is disclosed for obtaining a three-dimensional image of a substrate using a linescan camera. The camera is trained on the substrate normal to its surface to sense the intensity of light reflected from a thin strip of area running across the substrate surface along a first axis. A line of light, parallel to the first axis, is spanned or swept across a small region of the surface of the substrate in a direction perpendicular to the first axis to successively illuminate each of a plurality of strips of area lying in a band which includes the strip imaged by the camera. The linescan camera is synchronized to successively capture the image of the strip within its field of view each time a successive one of the strips in the band is spanned by the line of light. The successive images of the strip within the field of view of the camera are processed to establish the height (or depth) of the attributes, if any, lying within the strip imaged by the linescan camera. By imparting a relative motion between the linescan camera and the substrate, the height or depths of the attributes lying on the surface of the substrate can be found, thus allowing a three-dimensional image thereof to be established.
The three-dimensional imaging technique described in my '665 patent, while effective, nonetheless does incur certain disadvantages. In order to obtain the height or depth of the attributes lying in the particular strip of area imaged by the linescan camera, the image of the strip must be successively captured as the line of light is spanned across the substrate. The number of times the camera must successively capture the image of the strip within its field of view is dependent on the width of the band (i.e., the number of strips) spanned by the line of light. The number of strips in the band spanned by the line of light determines the range of height (or depth) that can be measured. To obtain a reasonable range of measurement, a large number of strips must be spanned by the line of light, which, in turn, requires that the linescan camera must successively image the same strip within its field of view a large number of times. Thus, the speed at which a three-dimensional image may be obtained using this technique is limited. Further, to practice this technique, a mechanism is required to repetitively sweep the line of light across the substrate surface.
Thus, there is a need for a technique for achieving a three-dimensional image of a substrate, using a linescan camera, which offers greater speed and requires less hardware than previous techniques.